The present invention relates to an ignition device for a rotary piston engine having a glow plug, and more particularly to a novel and improved structure to save power supplied to the glow plug and improve combustion.
The ignition in a rotary piston engine is usually carried out by two spark plugs which are called a leading plug and a trailing plug. However, since the rotary piston engine inherently has much internal recirculation of exhaust gas, misignition is apt to occur when the ignition is carried out by spark plugs, and in order to prevent misignition it is necessary to reduce the air-fuel ratio of the air-fuel mixture (or make the air-fuel mixture richer). This results in deterioration of the combustion efficiency.
The inventors of the present invention have made an extensive study to overcome the above difficulty and have found that combustion can be considerably improved even in a low speed-low load operation region where much recirculation of the exhaust gas is included, by using a glow plug instead of the conventional leading plug and hence the combustion efficiency is improved abd noxious exhaust gas components can be reduced. However, it also has been found that if the glow plug is continuously powered in order to enable the glow plug to ignite the air-fuel mixture over the entire operation range of the engine, a high electric power of more than 50 watts is required per glow plug if the glow plug is simply mounted on the engine housing. This is not practical for use in a commercial car. Further experiment has revealed that such a high power consumption is caused by the fact that an igniting portion of the glow plug is cooled too much because too much air-fuel mixture is directed to the igniting portion of the glow plug or the flow rate of the air-fuel mixture is too high around the igniting portion, and hence more electric power than that inherently required for the combustion of the air-fuel mixture is necessary.